Add predefined environment variables (DOKPLOY_FQDN, DOKPLOY_URL, etc.)

[Asamsig]

What problem will this feature address?

Currently, users must manually configure environment variables like NEXTAUTH_URL, when their application needs to know its own domain. This is error-prone as users must duplicate the domain configuration in multiple places, and it's easy to forget to update these when changing domains.

Describe the solution you'd like

Add automatic predefined environment variables for example:

Variable	Description
DOKPLOY_FQDN	Domain host (e.g., example.com)
DOKPLOY_URL	Full URL with protocol (e.g., https://example.com)
DOKPLOY_PORT	Port from domain config
DOKPLOY_BRANCH	Git branch being deployed
DOKPLOY_COMMIT_SHA	Git commit SHA
DOKPLOY_IMAGE_TAG	Docker image tag (for docker source apps)
DOKPLOY_IMAGE_DIGEST	Docker image digest
DOKPLOY_APPLICATION_ID	Application ID
DOKPLOY_CONTAINER_NAME	Container/App name

These would be available at both build-time and runtime.

Implementation includes:

1. Add a primary switch to domains, this sets which domain to use for the environment variables
2. Should all domain names be made available somehow?

Describe alternatives you've considered

Manually configure these in environment variables (current approach)

Additional context

No response

Will you send a PR to implement it?

Yes